Simplified palette predictor update for video coding

ABSTRACT

An example device includes a memory configured to store at least a portion of an encoded video bitstream; and one or more processors that are implemented in circuitry and configured to: determine, based on a parameter of a first block of video data, a maximum number of entries to be used for palette-mode coding of the current block; generate, based on the determined maximum number of entries and based on a palette predictor, a palette for the first block of video data, the palette including one or more entries each including a palette index that is associated with a color value; decode, from the encoded video bitstream and for the first block of video data, index values for samples of the first block that identify entries in the palette; and reconstruct, based on the index values, the samples of the first block.

This application claims the benefit of U.S. Provisional Application No.62/905,105, filed Sep. 24, 2019, the entire contents of which areincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), andextensions of such standards. The video devices may transmit, receive,encode, decode, and/or store digital video information more efficientlyby implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

In general this disclosure describes techniques for palette mode codingof video data. Performing palette mode coding of video data may involvecopying of entries between palettes, such as copying entries from afirst palette to a second palette. For instance, a video coder may copyentries from a predictor palette to a palette for a current block ofvideo data. Each copy operation may consume system resources such asmemory, processing power, and battery life in the case of a mobile orother battery-powered device. As such, it may be desirable to reduce thenumber of copy operations performed. The number of copy operations maybe positively related to a number of entries in a palette. As such,reducing the number of entries in a palette may reduce the number ofcopy operations performed. However, reducing a maximum number of entriesin a palette may reduce the efficiency of palette-mode coding, which maynot be desirable.

In accordance with one or more techniques of this disclosure, a videocoder may dynamically adjust a maximum number of entries to be used forpalette-mode coding. For instance, based on a parameter of a first blockof video data, a video coder may determine a number of entries to beused for palette-mode coding of the current block. As one specificexample, the video coder may restrict the maximum number of entries to afirst value (e.g., 16) based on the parameter having a first value or toa second value (e.g., 32) based on the parameter having a second value.Hence, in some examples, the second value is greater than the firstvalue. In this way, the video coder may reduce the amount of systemresources used for palette-mode coding. For instance, the video codermay reduce the number of copy operations without unduly reducing theefficiency of palette-mode coding.

In one example, a device includes a memory configured to store at leasta portion of an encoded video bitstream; and one or more processors thatare implemented in circuitry and configured to: determine, based on aparameter of a first block of video data, a maximum number of entries tobe used for palette-mode coding of the current block; generate, based onthe determined maximum number of entries and based on a palettepredictor, a palette for the first block of video data, the paletteincluding one or more entries each including a palette index that isassociated with a color value; decode, from the encoded video bitstreamand for the first block of video data, index values for samples of thefirst block that identify entries in the palette; and reconstruct, basedon the index values, the samples of the first block.

In another example, a method includes determining, based on a parameterof a first block of video data, a maximum number of entries to be usedfor palette-mode coding of the current block; generating, based on thedetermined maximum number of entries and based on a palette predictor, apalette for the first block of video data, the palette including one ormore entries each including a palette index that is associated with acolor value; decoding, from an encoded video bitstream and for the firstblock of video data, index values for samples of the first block thatidentify entries in the palette; and reconstructing, based on the indexvalues, the samples of the first block.

In another example, a device includes a memory configured to store atleast a portion of an encoded video bitstream; and one or moreprocessors that are implemented in circuitry and configured to:determine, based on a parameter of a first block of video data, amaximum number of entries to be used for palette-mode coding of thecurrent block; generate, based on the determined maximum number ofentries and based on a palette predictor, a palette for the first blockof video data, the palette including one or more entries each includinga palette index that is associated with a color value; and encode, inthe encoded video bitstream and for the first block of video data, indexvalues for samples of the first block that identify entries in thepalette.

In another example, a method includes determining, based on a parameterof a first block of video data, a maximum number of entries to be usedfor palette-mode coding of the current block; generating, based on thedetermined maximum number of entries and based on a palette predictor, apalette for the first block of video data, the palette including one ormore entries each including a palette index that is associated with acolor value; and encoding, in an encoded video bitstream and for thefirst block of video data, index values for samples of the first blockthat identify entries in the palette.

In another example, a computer-readable storage medium storesinstructions that, when executed, cause one or more processors of avideo encoder to: determine, based on a parameter of a first block ofvideo data, a maximum number of entries to be used for palette-modecoding of the current block; generate, based on the determined maximumnumber of entries and based on a palette predictor, a palette for thefirst block of video data, the palette including one or more entrieseach including a palette index that is associated with a color value;and encode, in an encoded video bitstream and for the first block ofvideo data, index values for samples of the first block that identifyentries in the palette.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages of the disclosure will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure, and a corresponding coding tree unit(CTU).

FIG. 3 is a block diagram illustrating an example video encoder that mayperform the techniques of this disclosure.

FIG. 4 is a block diagram illustrating an example video decoder that mayperform the techniques of this disclosure.

FIG. 5 is a conceptual diagram illustrating an example of determining apalette for coding video data, consistent with techniques of thisdisclosure.

FIG. 6 is a conceptual diagram illustrating an example of determiningindices to a palette for a block of pixels, consistent with techniquesof this disclosure.

FIG. 7 is a conceptual diagram illustrating example coding of indicesusing horizontal and vertical traverse scans.

FIGS. 8 and 9 are conceptual diagrams illustrating palette tablederivation and updating of a palette predictor.

FIG. 10 is a conceptual diagram illustrating an example of using onlythe first W×H entries in a palette predictor for predicting the palettepredictor.

FIG. 11 is a conceptual diagram illustrating an example of using onlythe first W×H entries in a palette predictor for predicting the palettepredictor where stuffing is restricted to the first W×H elements.

FIG. 12 is a flowchart illustrating an example method for decoding acurrent block of video data.

FIG. 13 is a flowchart illustrating an example method for decoding acurrent block of video data.

FIG. 14 is a flowchart illustrating an example method for coding a blockusing palette-mode compression, in accordance with one or moretechniques of this disclosure.

DETAILED DESCRIPTION

This disclosure describes techniques for video coding and compression.In particular, this disclosure describes techniques for palette-basedcoding of video data. For instance, this disclosure describes techniquesto support coding of video content, especially screen content withpalette coding, such as techniques for improved palette construction,and techniques for signaling for palette coding.

In traditional video coding, images are assumed to be continuous-toneand spatially smooth. Based on these assumptions, various tools havebeen developed such as block-based transform, filtering, etc., and suchtools have shown good performance for natural content videos.

However, in applications like remote desktop, collaborative work andwireless display, computer generated screen content may be the dominantcontent to be compressed. This type of content tends to havediscrete-tone and feature sharp lines, and high contrast objectboundaries. The assumption of continuous-tone and smoothness may nolonger apply and thus traditional video coding techniques may not beefficient ways to compress video data.

Based on the characteristics of screen content video, palette coding isintroduced to improve screen content coding (SCC) efficiency as proposedin Guo et al., “Palette Mode for Screen Content Coding,” JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 andISO/IEC JTC 1/SC 29/WG 11, 13th Meeting: Incheon, KR, 18-26 Apr. 2013,Document: JCTVC-M0323, available athttp://phenix.it-sudparis.eu/jct/doc_end_user/documents/13_Incheon/wg11/aCTVC-M0323-v3.zip,(hereinafter “JCTVC-M0323”). Specifically, palette coding introduces alookup table, i.e., a color palette, to compress repetitive pixel valuesbased on the fact that in SCC, colors within one CU usually concentrateon a few peak values. Given a palette for a specific CU, pixels withinthe CU are mapped to palette indices. In the second stage, an effectivecopy from left run length method is proposed to effectively compress theindex block's repetitive pattern. In some examples, the palette indexcoding mode may be generalized to both copy from left and copy fromabove with run length coding. Note that, in some examples, notransformation process may be invoked for palette coding to avoidblurring sharp edges which can have a huge negative impact on visualquality of screen contents.

As discussed above, this disclosure describes palette-based coding,which may be particularly suitable for screen generated content coding.For example, assume a particular area of video data has a relativelysmall number of colors. A video coder (a video encoder or video decoder)may code a so-called “palette” as a table of colors for representing thevideo data of the particular area (e.g., a given block). Each pixel maybe associated with an entry in the palette that represents the color ofthe pixel. For example, the video coder may code an index that maps thepixel value to the appropriate value in the palette.

In the example above, a video encoder may encode a block of video databy determining a palette for the block, locating an entry in the paletteto represent the color value of each pixel, and encoding the palettewith index values for the pixels mapping the pixel value to the palette.A video decoder may obtain, from an encoded bitstream, a palette for ablock, as well as index values for the pixels of the block. The videodecoder may map the index values of the pixels to entries of the paletteto reconstruct the luma and chroma pixel values of the block.

The example above is intended to provide a general description ofpalette-based coding. In various examples, the techniques described inthis disclosure may include techniques for various combinations of oneor more of signaling palette-based coding modes, transmitting palettes,predicting palettes, deriving palettes, and transmitting palette-basedcoding maps and other syntax elements. Such techniques may improve videocoding efficiency, e.g., requiring fewer bits to represent screengenerated content.

For example, according to aspects of this disclosure, a video coder(video encoder or video decoder) may code one or more syntax elementsfor each block that is coded using a palette coding mode. For example,the video coder may code a palette_mode_flag to indicate whether apalette-based coding mode is to be used for coding a particular block.In this example, a video encoder may encode a palette_mode_flag with avalue that is equal to one to specify that the block currently beingencoded (“current block”) is encoded using a palette mode. In this case,a video decoder may obtain the palette_mode_flag from the encodedbitstream and apply the palette-based coding mode to decode the block.In instances in which there is more than one palette-based coding modeavailable (e.g., there is more than one palette-based techniqueavailable for coding), one or more syntax elements may indicate one of aplurality of different palette modes for the block.

In some instances, the video encoder may encode a palette_mode_flag witha value that is equal to zero to specify that the current block is notencoded using a palette mode. In such instances, the video encoder mayencode the block using any of a variety of inter-predictive,intra-predictive, or other coding modes. When the palette_mode_flag isequal to zero, the video encoder may encode additional information(e.g., syntax elements) to indicate the specific mode that is used forencoding the respective block. In some examples, as described below, themode may be an HEVC coding mode. The use of the palette_mode_flag isdescribed for purposes of example. In other examples, other syntaxelements such as multi-bit codes may be used to indicate whether thepalette-based coding mode is to be used for one or more blocks, or toindicate which of a plurality of modes are to be used.

When a palette-based coding mode is used, a palette may be transmittedby an encoder in the encoded video data bitstream for use by a decoder.A palette may be transmitted for each block or may be shared among anumber of blocks in a picture or slice. The palette may refer to anumber of pixel values that are dominant and/or representative for theblock, including, e.g., a luma value and two chroma values.

In some examples, a syntax element, such as a transpose flag, may becoded to indicate whether a transpose process is applied to paletteindices of a current palette. If the transpose flag is zero, the paletteindices for samples may be coded in a horizontal traverse scan.Similarly, if the transpose flag is one, the palette indices for samplesmay be coded in a vertical traverse scan. This can be thought of asdecoding the index values assuming horizontal traverse scan and thentransposing the block (rows to columns).

As discussed above, palette coding is designed to handle the clusteringcolours for screen contents. Palette coding employs base colours and anindex map to represent the input image block. A flag may be transmittedfor each Coding unit (CU) to signal whether the palette mode is used inthe current CU. If the palette mode is utilized, the pixels values inthe CU are represented by a small set of representative color values.The set is referred to as the palette. For pixels with values close tothe palette colors, the palette indices are signalled. For pixels withvalues outside the palette, the pixel is denoted with an escape symboland the quantized pixel values are signaled directly.

To decode a palette encoded block, the decoder needs to decode palettecolors and indices. Palette colors are described by a palette table andencoded by palette table coding tools. An escape flag is signaled foreach CU to indicate if escape symbols are present in the current CU. Ifescape symbols are present, the palette table is augmented by one andthe last index is assigned to the escape mode. Palette indices of allpixels in a CU form a palette index map and are encoded by palette indexmap coding tools.

For coding the palette index map, the video coder may code the indicesusing horizontal and vertical traverse scans. FIG. 7 is a conceptualdiagram illustrating example coding of indices using horizontal andvertical traverse scans.

The palette indices are coded using two main palette sample modes:‘INDEX’ and ‘COPY_ABOVE’. The mode is signalled using a flag except forthe top row when horizontal scan is used, the first column when thevertical scan is used, or when the previous mode was ‘COPY_ABOVE’. Inthe ‘COPY_ABOVE’ mode, the palette index of the sample in the row aboveis copied. In the ‘INDEX’ mode, the palette index is explicitlysignalled. For both ‘INDEX’ and ‘COPY_ABOVE’ modes, a run value issignalled which specifies the number pixels that are coded using thesame mode.

The encoding order for index map may be as follows: First, the number ofindex values for the CU is signalled. This is followed by signalling ofthe actual index values for the entire CU using truncated binary coding.Both the number of indices as well as the index values are coded inbypass mode. This groups the index-related bypass bins together. Thenthe palette mode (INDEX or COPY_ABOVE) and run are signalled in aninterleaved manner. Finally, the component escape values correspondingto the escape samples for the entire CU are grouped together and codedin bypass mode. An additional syntax element (e.g., last_run_type_flag)may be signalled after signalling the index values. This syntax element,in conjunction with the number of indices, may eliminate the need tosignal the run value corresponding to the last run in the block.

In the 15th JVET meeting in Gothenburg, Sweden, palette mode was adoptedinto Versatile Video coding (VVC) for YUV4:4:4 format. The palette modesyntax is the same as in HEVC SCM (see e.g., R. Joshi, J. Xu, R. Cohen,S. Liu, Y. Ye, “Screen Content Coding Test Model 7 Encoder Description(SCM 7)”, JCTVC-W1014, 2016; and R. Joshi, S. Liu, G. J. Sullivan, Y.-K.Wang, J. Xu, Y. Ye, “HEVC Screen Content Coding Draft Text 6”,JCTVC-W1005, 2016) with modification of palette mode signaling and withinclusion of separated palette mode (separated palettepredictor/table/syntax parsing for luma coding tree and chroma codingtree) for slices using dual tree for luma and chroma components.

In palette mode, the video coder (e.g., video encoder 200 and/or videodecoder 300 as described below, e.g., with reference to FIGS. 1, 3 and4) may code a flag for each CU to signal whether the palette mode isused in the current CU (e.g., palette_mode_flag). The maximum CU sizeallowed for palette mode is size 64×64 (maximum CU size in HEVC). If thepalette mode is utilized, the pixels values in the CU are represented bya small set of representative color values. The set may be referred toas the palette. The video coder may signal palette indices for pixelswith values close to the palette colors. The video coder may denotepixels with values outside the palette with an escape symbol, and maysignal the quantized pixel values directly.

To decode a palette encoded block, the decoder may decode palette colorsand indices. Palette colors are described by a palette table and encodedby palette table coding tools. The video coder may signal an escape flagfor each CU to indicate if escape symbols are present in the current CU.If escape symbols are present, the video coder may augment the palettetable by one and assign the last index to the escape mode. Paletteindices of all pixels in a CU form a palette index map and are encodedby palette index map coding tools.

The video coder may maintain a palette predictor for coding of thepalette table. The video coder may periodically initialize the palettepredictor. For instance, the video coder may initialize the palettepredictor at the beginning of each slice. In some examples, toinitialize the palette predictor, the video coder may reset the palettepredictor to 0. The video coder may signal a reuse flag for each entryin the palette predictor to indicate whether it is part of the currentpalette. In some examples, the video coder may code the reuse flagsusing run-length coding of zeros. The video coder may signal the numberof new palette entries using an exponential Golomb code of order 0. Thevideo coder may signal the component values for the new palette entries.After encoding the current CU, the video coder may update the palettepredictor using the current palette. In some examples, the video codermay add entries from the previous palette predictor which are not reusedin the current palette at the end of a new palette predictor until themaximum size allowed is reached. The adding of entries from the previouspalette predictor may be referred to as palette stuffing.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,uncoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1, system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Source device 102 and destination device 116 may comprise any of awide range of devices, including desktop computers, notebook (i.e.,laptop) computers, tablet computers, set-top boxes, telephone handsetssuch smartphones, televisions, cameras, display devices, digital mediaplayers, video gaming consoles, video streaming device, or the like. Insome cases, source device 102 and destination device 116 may be equippedfor wireless communication, and thus may be referred to as wirelesscommunication devices.

In the example of FIG. 1, source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the techniques for palette modecoding. Thus, source device 102 represents an example of a videoencoding device, while destination device 116 represents an example of avideo decoding device. In other examples, a source device and adestination device may include other components or arrangements. Forexample, source device 102 may receive video data from an external videosource, such as an external camera. Likewise, destination device 116 mayinterface with an external display device, rather than including anintegrated display device.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform techniques forpalette mode coding. Source device 102 and destination device 116 aremerely examples of such coding devices in which source device 102generates coded video data for transmission to destination device 116.This disclosure refers to a “coding” device as a device that performscoding (encoding and/or decoding) of data. Thus, video encoder 200 andvideo decoder 300 represent examples of coding devices, in particular, avideo encoder and a video decoder, respectively. In some examples,devices 102, 116 may operate in a substantially symmetrical manner suchthat each of devices 102, 116 include video encoding and decodingcomponents. Hence, system 100 may support one-way or two-way videotransmission between video devices 102, 116, e.g., for video streaming,video playback, video broadcasting, or video telephony.

In general, video source 104 represents a source of video data (i.e.,raw, uncoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some example, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although shown separately from video encoder 200 and videodecoder 300 in this example, it should be understood that video encoder200 and video decoder 300 may also include internal memories forfunctionally similar or equivalent purposes. Furthermore, memories 106,120 may store encoded video data, e.g., output from video encoder 200and input to video decoder 300. In some examples, portions of memories106, 120 may be allocated as one or more video buffers, e.g., to storeraw, decoded, and/or encoded video data.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may modulatethe received transmission signal, according to a communication standard,such as a wireless communication protocol. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 116. Similarly, destination device 116may access encoded data from storage device 116 via input interface 122.Storage device 116 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video generated by source device 102. Destination device 116may access stored video data from file server 114 via streaming ordownload. File server 114 may be any type of server device capable ofstoring encoded video data and transmitting that encoded video data tothe destination device 116. File server 114 may represent a web server(e.g., for a website), a File Transfer Protocol (FTP) server, a contentdelivery network device, or a network attached storage (NAS) device.Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., DSL, cable modem, etc.), or acombination of both that is suitable for accessing encoded video datastored on file server 114. File server 114 and input interface 122 maybe configured to operate according to a streaming transmission protocol,a download transmission protocol, or a combination thereof.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receiver, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., storage device 112,file server 114, or the like). The encoded video bitstreamcomputer-readable medium 110 may include signaling information definedby video encoder 200, which is also used by video decoder 300, such assyntax elements having values that describe characteristics and/orprocessing of video blocks or other coded units (e.g., slices, pictures,groups of pictures, sequences, or the like). Display device 118 displaysdecoded pictures of the decoded video data to a user. Display device 118may represent any of a variety of display devices such as a cathode raytube (CRT), a liquid crystal display (LCD), a plasma display, an organiclight emitting diode (OLED) display, or another type of display device.

Although not shown in FIG. 1, in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream. If applicable, MUX-DEMUX unitsmay conform to the ITU H.223 multiplexer protocol, or other protocolssuch as the user datagram protocol (UDP).

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as the Joint Exploration TestModel (JEM) or ITU-T H.266, also referred to as Versatile Video Coding(VVC). A recent draft of the VVC standard is described in Bross, et al.“Versatile Video Coding (Draft 6),” Joint Video Experts Team (JVET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 15^(th) Meeting:Gothenburg, SE, 3-12 Jul. 2019, JVET-O2001-v14 (hereinafter “VVC Draft6”). The techniques of this disclosure, however, are not limited to anyparticular coding standard.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to JEM or VVC. According to JEM or VVC,a video coder (such as video encoder 200) partitions a picture into aplurality of coding tree units (CTUs). Video encoder 200 may partition aCTU according to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) partitions. A triple tree partition is apartition where a block is split into three sub-blocks. In someexamples, a triple tree partition divides a block into three sub-blockswithout dividing the original block through the center. The partitioningtypes in MTT (e.g., QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, orother partitioning structures. For purposes of explanation, thedescription of the techniques of this disclosure is presented withrespect to QTBT partitioning. However, it should be understood that thetechniques of this disclosure may also be applied to video codersconfigured to use quadtree partitioning, or other types of partitioningas well.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of JEM and VVC also provide an affine motion compensationmode, which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofJEM and VVC provide sixty-seven intra-prediction modes, includingvarious directional modes, as well as planar mode and DC mode. Ingeneral, video encoder 200 selects an intra-prediction mode thatdescribes neighboring samples to a current block (e.g., a block of a CU)from which to predict samples of the current block. Such samples maygenerally be above, above and to the left, or to the left of the currentblock in the same picture as the current block, assuming video encoder200 codes CTUs and CUs in raster scan order (left to right, top tobottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of thecoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) coefficients at the front of the vector and to place lowerenergy (and therefore higher frequency) transform coefficients at theback of the vector. In some examples, video encoder 200 may utilize apredefined scan order to scan the quantized transform coefficients toproduce a serialized vector, and then entropy encode the quantizedtransform coefficients of the vector. In other examples, video encoder200 may perform an adaptive scan. After scanning the quantized transformcoefficients to form the one-dimensional vector, video encoder 200 mayentropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information of apicture into CTUs, and partitioning of each CTU according to acorresponding partition structure, such as a QTBT structure, to defineCUs of the CTU. The syntax elements may further define prediction andresidual information for blocks (e.g., CUs) of video data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block.

This disclosure may generally refer to “signaling” certain information,such as syntax elements. The term “signaling” may generally refer to thecommunication of values syntax elements and/or other data used to decodeencoded video data. That is, video encoder 200 may signal values forsyntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

FIGS. 2A and 2B are conceptual diagram illustrating an example quadtreebinary tree (QTBT) structure 130, and a corresponding coding tree unit(CTU) 132. The solid lines represent quadtree splitting, and dottedlines indicate binary tree splitting. In each split (i.e., non-leaf)node of the binary tree, one flag is signaled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting in this example.For the quadtree splitting, there is no need to indicate the splittingtype, since quadtree nodes split a block horizontally and verticallyinto 4 sub-blocks with equal size. Accordingly, video encoder 200 mayencode, and video decoder 300 may decode, syntax elements (such assplitting information) for a region tree level of QTBT structure 130(i.e., the solid lines) and syntax elements (such as splittinginformation) for a prediction tree level of QTBT structure 130 (i.e.,the dashed lines). Video encoder 200 may encode, and video decoder 300may decode, video data, such as prediction and transform data, for CUsrepresented by terminal leaf nodes of QTBT structure 130.

In general, CTU 132 of FIG. 2B may be associated with parametersdefining sizes of blocks corresponding to nodes of QTBT structure 130 atthe first and second levels. These parameters may include a CTU size(representing a size of CTU 132 in samples), a minimum quadtree size(MinQTSize, representing a minimum allowed quadtree leaf node size), amaximum binary tree size (MaxBTSize, representing a maximum allowedbinary tree root node size), a maximum binary tree depth (MaxBTDepth,representing a maximum allowed binary tree depth), and a minimum binarytree size (MinBTSize, representing the minimum allowed binary tree leafnode size).

The root node of a QTBT structure corresponding to a CTU may have fourchild nodes at the first level of the QTBT structure, each of which maybe partitioned according to quadtree partitioning. That is, nodes of thefirst level are either leaf nodes (having no child nodes) or have fourchild nodes. The example of QTBT structure 130 represents such nodes asincluding the parent node and child nodes having solid lines forbranches. If nodes of the first level are not larger than the maximumallowed binary tree root node size (MaxBTSize), they can be furtherpartitioned by respective binary trees. The binary tree splitting of onenode can be iterated until the nodes resulting from the split reach theminimum allowed binary tree leaf node size (MinBTSize) or the maximumallowed binary tree depth (MaxBTDepth). The example of QTBT structure130 represents such nodes as having dashed lines for branches. Thebinary tree leaf node is referred to as a coding unit (CU), which isused for prediction (e.g., intra-picture or inter-picture prediction)and transform, without any further partitioning. As discussed above, CUsmay also be referred to as “video blocks” or “blocks.”

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 (luma samples and two corresponding 64×64 chroma samples),the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, theMinBTSize (for both width and height) is set as 4, and the MaxBTDepth isset as 4. The quadtree partitioning is applied to the CTU first togenerate quad-tree leaf nodes. The quadtree leaf nodes may have a sizefrom 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If theleaf quadtree node is 128×128, it will not be further split by thebinary tree, since the size exceeds the MaxBTSize (i.e., 64×64, in thisexample). Otherwise, the leaf quadtree node will be further partitionedby the binary tree. Therefore, the quadtree leaf node is also the rootnode for the binary tree and has the binary tree depth as 0. When thebinary tree depth reaches MaxBTDepth (4, in this example), no furthersplitting is permitted. When the binary tree node has width equal toMinBTSize (4, in this example), it implies no further horizontalsplitting is permitted. Similarly, a binary tree node having a heightequal to MinBTSize implies no further vertical splitting is permittedfor that binary tree node. As noted above, leaf nodes of the binary treeare referred to as CUs, and are further processed according toprediction and transform without further partitioning.

FIG. 3 is a block diagram illustrating an example video encoder 200 thatmay perform the techniques of this disclosure. FIG. 3 is provided forpurposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200 inthe context of video coding standards such as the HEVC video codingstandard and the H.266 video coding standard in development. However,the techniques of this disclosure are not limited to these video codingstandards, and are applicable generally to video encoding and decoding.

In the example of FIG. 3, video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220. Any or all of video data memory 230, mode selection unit 202,residual generation unit 204, transform processing unit 206,quantization unit 208, inverse quantization unit 210, inverse transformprocessing unit 212, reconstruction unit 214, filter unit 216, DPB 218,and entropy encoding unit 220 may be implemented in one or moreprocessors or in processing circuitry. Moreover, video encoder 200 mayinclude additional or alternative processors or processing circuitry toperform these and other functions.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 3 are illustrated to assist with understandingthe operations performed by video encoder 200. The units may beimplemented as fixed-function circuits, programmable circuits, or acombination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that canprogrammed to perform various tasks, and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, the one or more of the units maybe distinct circuit blocks (fixed-function or programmable), and in someexamples, the one or more units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1) may store theobject code of the software that video encoder 200 receives andexecutes, or another memory within video encoder 200 (not shown) maystore such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 202 may partition a CTU of thepicture in accordance with a tree structure, such as the QTBT structureor the quad-tree structure of HEVC described above. As described above,video encoder 200 may form one or more CUs from partitioning a CTUaccording to the tree structure. Such a CU may also be referred togenerally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,uncoded version of the current block from video data memory 230 and theprediction block from mode selection unit 202. Residual generation unit204 calculates sample-by-sample differences between the current blockand the prediction block. The resulting sample-by-sample differencesdefine a residual block for the current block. In some examples,residual generation unit 204 may also determine differences betweensample values in the residual block to generate a residual block usingresidual differential pulse code modulation (RDPCM). In some examples,residual generation unit 204 may be formed using one or more subtractorcircuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder200 and video decoder 300 may also support asymmetric partitioning forPU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction.

In examples where mode selection unit does not further partition a CUinto PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 300 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as an intra-block copy modecoding, an affine-mode coding, and linear model (LM) mode coding, as fewexamples, mode selection unit 202, via respective units associated withthe coding techniques, generates a prediction block for the currentblock being encoded. In some examples, such as palette mode coding, modeselection unit 202 may not generate a prediction block, and insteadgenerate syntax elements that indicate the manner in which toreconstruct the block based on a selected palette. In such modes, modeselection unit 202 may provide these syntax elements to entropy encodingunit 220 to be encoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block.

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform. Insome examples, transform processing unit 206 does not apply transformsto a residual block.

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the coefficient blocks associated withthe current block by adjusting the QP value associated with the CU.Quantization may introduce loss of information, and thus, quantizedtransform coefficients may have lower precision than the originaltransform coefficients produced by transform processing unit 206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Reconstruction unit 214 mayproduce a reconstructed block corresponding to the current block (albeitpotentially with some degree of distortion) based on the reconstructedresidual block and a prediction block generated by mode selection unit202. For example, reconstruction unit 214 may add samples of thereconstructed residual block to corresponding samples from theprediction block generated by mode selection unit 202 to produce thereconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Operations of filter unit 216 may be skipped, in some examples.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 216 are not needed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 216 are needed, filter unit 216may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200. Forexample, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying a MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding blocks andthe chroma coding blocks.

As shown in FIG. 3, mode selection unit 202 may include a paletteprediction unit 227, which may be configured to perform videocompression using palette-mode encoding. To code a current block ofvideo data using palette-mode encoding, palette prediction unit 227 maygenerate a palette for the current block. The palette may includeentries with color values that correspond to the mostly commonly usedcolors in the current block (e.g., determined using a histogram).Palette prediction unit 227 may encode a representation of the paletteand index values for samples of the current block that map to entries inthe palette.

In accordance with one or more techniques of this disclosure, paletteprediction unit 227 may dynamically adjust a maximum number of entriesto be used for palette-mode coding. For instance, based on a parameterof a first block of video data, palette prediction unit 227 maydetermine a number of entries to be used for palette-mode coding of thecurrent block. As one specific example, palette prediction unit 227 mayrestrict the maximum number of entries to a first value (e.g., 16) basedon the parameter having a first value or to a second value (e.g., 32),different that the first value, based on the parameter having a secondvalue. Hence, in some examples, the second value is greater than thefirst value. In other words, palette prediction unit 227 may restrictthe maximum number of entries to be either 16 or 32. In this way,palette prediction unit 227 may reduce the amount of system resourcesused for palette-mode coding.

Video encoder 200 represents an example of a device configured to encodevideo data including a memory configured to store video data, and one ormore processing units implemented in circuitry and configured todetermine, based on a parameter of a first block of video data, amaximum number of entries to be used for palette-mode coding of thecurrent block; generate, based on the determined maximum size and basedon a palette predictor, a palette for the first block of video data, thepalette including one or more entries each including a palette indexthat is associated with a color value; decode, from a coded videobitstream and for the first block of video data, index values forsamples of the first block that identify entries in the palette; andreconstruct, based on the index values, the samples of the first block.

FIG. 4 is a block diagram illustrating an example video decoder 300 thatmay perform the techniques of this disclosure. FIG. 4 is provided forpurposes of explanation and is not limiting on the techniques as broadlyexemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 is describedaccording to the techniques of JEM, VVC, and HEVC. However, thetechniques of this disclosure may be performed by video coding devicesthat are configured to other video coding standards.

In the example of FIG. 4, video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Any or all of CPB memory 320, entropydecoding unit 302, prediction processing unit 304, inverse quantizationunit 306, inverse transform processing unit 308, reconstruction unit310, filter unit 312, and DPB 314 may be implemented in one or moreprocessors or in processing circuitry. Moreover, video decoder 300 mayinclude additional or alternative processors or processing circuitry toperform these and other functions.

Prediction processing unit 304 includes motion compensation unit 316 andintra-prediction unit 318. Prediction processing unit 304 may includeaddition units to perform prediction in accordance with other predictionmodes. As examples, prediction processing unit 304 may include a paletteunit, an intra-block copy unit (which may form part of motioncompensation unit 316), an affine unit, a linear model (LM) unit, or thelike. In other examples, video decoder 300 may include more, fewer, ordifferent functional components.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. CPB memory 320and DPB 314 may be provided by the same memory device or separate memorydevices. In various examples, CPB memory 320 may be on-chip with othercomponents of video decoder 300, or off-chip relative to thosecomponents.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to executed by processing circuitry of video decoder 300.

The various units shown in FIG. 4 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 3, fixed-function circuits referto circuits that provide particular functionality, and are preset on theoperations that can be performed. Programmable circuits refer tocircuits that can programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, the one ormore of the units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, the one or more units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the coefficient block.

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 3).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Again, intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 3).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Filter unit 312 may perform one or more filter operations onreconstructed blocks. For example, filter unit 312 may performdeblocking operations to reduce blockiness artifacts along edges of thereconstructed blocks. Operations of filter unit 312 are not necessarilyperformed in all examples.

Video decoder 300 may store the reconstructed blocks in DPB 314. Asdiscussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures fromDPB for subsequent presentation on a display device, such as displaydevice 118 of FIG. 1.

As shown in FIG. 4, prediction processing unit 304 may include a paletteprediction unit 319, which may be configured to perform videocompression using palette-mode decoding. To decode a current block ofvideo data using palette-mode encoding, palette prediction unit 319 maygenerate a palette for the current block. The palette may includeentries with color values that correspond to the mostly commonly usedcolors in the current block (e.g., determined using a histogram).Palette prediction unit 319 may decode a representation of index valuesfor samples of the current block that map to entries in the palette.Palette prediction unit 319 may reconstruct the samples of the currentblock based on the index values (e.g., and any separately signalledescape samples).

In accordance with one or more techniques of this disclosure, paletteprediction unit 319 may dynamically adjust a maximum number of entriesto be used for palette-mode coding. For instance, based on a parameterof a first block of video data, palette prediction unit 319 maydetermine a number of entries to be used for palette-mode coding of thecurrent block. As one specific example, palette prediction unit 319 mayrestrict the maximum number of entries to a first value (e.g., 16) basedon the parameter having a first value or to a second value (e.g., 32),different than the first value, based on the parameter having a secondvalue. In other words, palette prediction unit 319 may restrict themaximum number of entries to be either 16 or 32. Hence, in someexamples, the second value is greater than the first value. In this way,palette prediction unit 319 may reduce the amount of system resourcesused for palette-mode coding.

In this manner, video decoder 300 represents an example of a videodecoding device including a memory configured to store video data, andone or more processing units implemented in circuitry and configured todetermine, based on a parameter of a first block of video data, amaximum number of entries to be used for palette-mode coding of thecurrent block; generate, based on the determined maximum size and basedon a palette predictor, a palette for the first block of video data, thepalette including one or more entries each including a palette indexthat is associated with a color value; decode, from a coded videobitstream and for the first block of video data, index values forsamples of the first block that identify entries in the palette; andreconstruct, based on the index values, the samples of the first block.

FIG. 5 is a conceptual diagram illustrating an example of determining apalette for coding video data, consistent with techniques of thisdisclosure. The example of FIG. 5 includes a picture 1780 having a firstcoding unit (CU) 1800 that is associated with first palettes 1840 and asecond CU 1880 that is associated with second palettes 1920. Asdescribed in greater detail below and in accordance with the techniquesof this disclosure, second palettes 1920 are based on first palettes1840. Picture 1780 also includes block 1960 coded with anintra-prediction coding mode and block 2000 that is coded with aninter-prediction coding mode.

The techniques of FIG. 5 are described in the context of video encoder200 (FIG. 1 and FIG. 2) and video decoder 300 (FIG. 1 and FIG. 4) andwith respect to the HEVC Standard for purposes of explanation. However,it should be understood that the techniques of this disclosure are notlimited in this way, and may be applied by other video coding processorsand/or devices in other video coding processes and/or standards (e.g.,VVC).

In general, a palette refers to a number of pixel values that aredominant and/or representative for a CU currently being coded, such asCU 1880 in the example of FIG. 5. First palettes 1840 and secondpalettes 1920 are shown as including multiple palettes. In someexamples, a video coder (such as video encoder 200 or video decoder 300)may code palettes separately for each color component of a CU. Forexample, video encoder 200 may encode a palette for a luma (Y) componentof a CU, another palette for a chroma (U) component of the CU, and yetanother palette for the chroma (V) component of the CU. In this example,entries of the Y palette may represent Y values of pixels of the CU,entries of the U palette may represent U values of pixels of the CU, andentries of the V palette may represent V values of pixels of the CU. Inanother example, video encoder 20 may encode a palette for a luma (Y)component of a CU, and another palette for two components (U, V) of theCU. In this example, entries of the Y palette may represent Y values ofpixels of the CU, and entries of the U-V palette may represent U-V valuepairs of pixels of the CU.

In other examples, video encoder 200 may encode a single palette for allcolor components of a CU. In this example, video encoder 200 may encodea palette having an i-th entry that is a triple value, including Yi, Ui,and Vi. In this case, the palette includes values for each of thecomponents of the pixels. Accordingly, the representation of palettes1840 and 1920 as a set of palettes having multiple individual palettesis merely one example and not intended to be limiting.

In the example of FIG. 5, first palettes 1840 includes three entries2020-2060 having entry index value 1, entry index value 2, and entryindex value 3, respectively. Entries 2020-2060 relate the index valuesto pixel values including pixel value A, pixel value B, and pixel valueC, respectively. As described herein, rather than coding the actualpixel values of first CU 180, a video coder (such as video encoder 200or video decoder 300) may use palette-based coding to code the pixels ofthe block using the indices 1-3. That is, for each pixel position offirst CU 1800, video encoder 200 may encode an index value for thepixel, where the index value is associated with a pixel value in one ormore of first palettes 1840. Video decoder 300 may obtain the indexvalues from a bitstream and reconstruct the pixel values using the indexvalues and one or more of first palettes 1840. Thus, first palettes 1840are transmitted by video encoder 200 in an encoded video data bitstreamfor use by video decoder 300 in palette-based decoding. In general, oneor more palettes may be transmitted for each CU or may be shared amongdifferent CUs.

Video encoder 200 and video decoder 300 may determine second palettes1920 based on first palettes 1840. For example, video encoder 200 mayencode a pred_palette_flag for each CU (including, as an example, secondCU 1880) to indicate whether the palette for the CU is predicted fromone or more palettes associated with one or more other CUs, such asneighboring CUs (spatially or based on scan order) or the most frequentsamples of a causal neighbor. For example, when the value of such a flagis equal to one, video decoder 300 may determine that second palettes1920 for second CU 1880 are predicted from one or more already decodedpalettes and therefore no new palettes for second CU 1880 are includedin a bitstream containing the pred_palette_flag. When such a flag isequal to zero, video decoder 300 may determine that palette 1920 forsecond CU 1880 is included in the bitstream as a new palette. In someexamples, pred_palette_flag may be separately coded for each differentcolor component of a CU (e.g., three flags, one for Y, one for U, andone for V, for a CU in YUV video). In other examples, a singlepred_palette_flag may be coded for all color components of a CU.

In the example above, the pred_palette_flag is signaled per-CU toindicate whether any of the entries of the palette for the current blockare predicted. In some examples, one or more syntax elements may besignaled on a per-entry basis. That is, a flag may be signaled for eachentry of a palette predictor to indicate whether that entry is presentin the current palette. As noted above, if a palette entry is notpredicted, the palette entry may be explicitly signaled.

When determining second palettes 1920 relative to first palettes 1840(e.g., pred_palette_flag is equal to one), video encoder 200 and/orvideo decoder 300 may locate one or more blocks from which thepredictive palettes, in this example first palettes 1840, aredetermined. The predictive palettes may be associated with one or moreneighboring CUs of the CU currently being coded (e.g., such asneighboring CUs (spatially or based on scan order) or the most frequentsamples of a causal neighbor), i.e., second CU 1880. The palettes of theone or more neighboring CUs may be associated with a predictor palette.In some examples, such as the example illustrated in FIG. 5, videoencoder 200 and/or video decoder 300 may locate a left neighboring CU,first CU 1800, when determining a predictive palette for second CU 1880.In other examples, video encoder 200 and/or video decoder 300 may locateone or more CUs in other positions relative to second CU 1880, such asan upper CU, CU 1960.

Video encoder 200 and/or video decoder 300 may determine a CU forpalette prediction based on a hierarchy. For example, video encoder 200and/or video decoder 300 may initially identify the left neighboring CU,first CU 1800, for palette prediction. If the left neighboring CU is notavailable for prediction (e.g., the left neighboring CU is coded with amode other than a palette-based coding mode, such as an intra-predictionmore or intra-prediction mode, or is located at the left-most edge of apicture or slice) video encoder 200 and/or video decoder 300 mayidentify the upper neighboring CU, CU 1960. Video encoder 200 and/orvideo decoder 300 may continue searching for an available CU accordingto a predetermined order of locations until locating a CU having apalette available for palette prediction. In some examples, videoencoder 200 and/or video decoder 300 may determine a predictive palettebased on multiple blocks and/or reconstructed samples of a neighboringblock.

While the example of FIG. 5 illustrates first palettes 1840 aspredictive palettes from a single CU, first CU 1800, in other examples,video encoder 200 and/or video decoder 300 may locate palettes forprediction from a combination of neighboring CUs. For example, videoencoder 200 and/or video decoder may apply one or more formulas,functions, rules or the like to generate a palette based on palettes ofone or a combination of a plurality of neighboring CUs.

In still other examples, video encoder 200 and/or video decoder 300 mayconstruct a candidate list including a number of potential candidatesfor palette prediction. A pruning process may be applied at both videoencoder 200 and video decoder 300 to remove duplicated candidates in thelist. In such examples, video encoder 200 may encode an index to thecandidate list to indicate the candidate CU in the list from which thecurrent CU used for palette prediction is selected (e.g., copies thepalette). Video decoder 300 may construct the candidate list in the samemanner, decode the index, and use the decoded index to select thepalette of the corresponding CU for use with the current CU.

In an example for purposes of illustration, video encoder 200 and videodecoder 300 may construct a candidate list that includes one CU that ispositioned above the CU currently being coded and one CU that ispositioned to the left of the CU currently being coded. In this example,video encoder 200 may encode one or more syntax elements to indicate thecandidate selection. For example, video encoder 200 may encode a flaghaving a value of zero to indicate that the palette for the current CUis copied from the CU positioned to the left of the current CU. Videoencoder 200 may encode the flag having a value of one to indicate thatthe palette for the current CU is copied from the CU positioned abovethe current CU. Video decoder 300 decodes the flag and selects theappropriate CU for palette prediction.

In still other examples, video encoder 200 and/or video decoder 300determine the palette for the CU currently being coded based on thefrequency with which sample values included in one or more otherpalettes occur in one or more neighboring CUs. For example, videoencoder 200 and/or video decoder 300 may track the colors associatedwith the most frequently used index values during coding of apredetermined number of CUs. Video encoder 200 and/or video decoder 300may include the most frequently used colors in the palette for the CUcurrently being coded.

In some examples, video encoder 200 and/or video decoder 300 may performentry-wise based palette prediction. For example, video encoder 200 mayencode one or more syntax elements, such as one or more flags, for eachentry of a predictive palette indicating whether the respectivepredictive palette entries are reused in the current palette (e.g.,whether pixel values in a palette of another CU are reused by thecurrent palette). In this example, video encoder 200 may encode a flaghaving a value equal to one for a given entry when the entry is apredicted value from a predictive palette (e.g., a corresponding entryof a palette associated with a neighboring CU). Video encoder 200 mayencode a flag having a value equal to zero for a particular entry toindicate that the particular entry is not predicted from a palette ofanother CU. In this example, video encoder 200 may also encodeadditional data indicating the value of the non-predicted palette entry.

In the example of FIG. 5, second palettes 1920 includes four entries2080-2140 having entry index value 1, entry index value 2, entry indexvalue 3, and entry index 4, respectively. Entries 2080-2140 relate theindex values to pixel values including pixel value A, pixel value B,pixel value C, and pixel value D, respectively. Video encoder 200 and/orvideo decoder 300 may use any of the above-described techniques tolocate first CU 1800 for purposes of palette prediction and copy entries1-3 of first palettes 184 to entries 1-3 of second palettes 1920 forcoding second CU 1880. In this way, video encoder 200 and/or videodecoder 300 may determine second palettes 192 based on first palettes184. In addition, video encoder 200 and/or video decoder 300 may codedata for entry 4 to be included with second palettes 1920. Suchinformation may include the number of palette entries not predicted froma predictor palette and the pixel values corresponding to those paletteentries.

In some examples, according to aspects of this disclosure, one or moresyntax elements may indicate whether palettes, such as second palettes1920, are predicted entirely from a predictive palette (shown in FIG. 5as first palettes 1840, but which may be composed of entries from one ormore blocks) or whether particular entries of second palettes 1920 arepredicted. For example, an initial syntax element may indicate whetherall of the entries are predicted. If the initial syntax elementindicates that not all of the entries are predicted (e.g., a flag havinga value of 0), one or more additional syntax elements may indicate whichentries of second palettes 1920 are predicted from the predictivepalette.

FIG. 6 is a conceptual diagram illustrating an example of determiningindices to a palette for a block of pixels, consistent with techniquesof this disclosure. For example, FIG. 6 includes a map 2400 of indexvalues (values 1, 2, and 3) that relate respective positions of pixelsassociated with the index values to an entry of palettes 2440. Palettes2440 may be determined in a similar manner as first palettes 1840 andsecond palettes 1920 described above with respect to FIG. 5.

Again, the techniques of FIG. 6 are described in the context of videoencoder 200 (FIG. 1 and FIG. 3) and video decoder 300 (FIG. 1 and FIG.4) and with respect to the HEVC video coding standard for purposes ofexplanation. However, it should be understood that the techniques ofthis disclosure are not limited in this way, and may be applied by othervideo coding processors and/or devices in other video coding processesand/or standards (e.g., VVC).

While map 2400 is illustrated in the example of FIG. 6 as including anindex value for each pixel position, it should be understood that inother examples, not all pixel positions may be associated with an indexvalue relating the pixel value to an entry of palettes 2440. That is, asnoted above, in some examples, video encoder 200 may encode (and videodecoder 300 may obtain, from an encoded bitstream) an indication of anactual pixel value (or its quantized version) for a position in map 2400if the pixel value is not included in palettes 2440.

In some examples, video encoder 200 and video decoder 300 may beconfigured to code an additional map indicating which pixel positionsare associated with index values. For example, assume that the (i, j)entry in the map corresponds to the (i, j) position of a CU. Videoencoder 200 may encode one or more syntax elements for each entry of themap (i.e., each pixel position) indicating whether the entry has anassociated index value. For example, video encoder 200 may encode a flaghaving a value of one to indicate that the pixel value at the (i, j)location in the CU is one of the values in palettes 2440. Video encoder200 may, in such an example, also encode a palette index (shown in theexample of FIG. 6 as values 1-3) to indicate that pixel value in thepalette and to allow video decoder to reconstruct the pixel value. Ininstances in which palettes 2440 include a single entry and associatedpixel value, video encoder 200 may skip the signaling of the indexvalue. Video encoder 200 may encode the flag to have a value of zero toindicate that the pixel value at the (i, j) location in the CU is notone of the values in palettes 2440. In this example, video encoder 200may also encode an indication of the pixel value for use by videodecoder 300 in reconstructing the pixel value. In some instances, thepixel value may be coded in a lossy manner.

The value of a pixel in one position of a CU may provide an indicationof values of one or more other pixels in other positions of the CU. Forexample, there may be a relatively high probability that neighboringpixel positions of a CU will have the same pixel value or may be mappedto the same index value (in the case of lossy coding, in which more thanone pixel value may be mapped to a single index value).

Accordingly, video encoder 200 may encode one or more syntax elementsindicating a number of consecutive pixels or index values in a givenscan order that have the same pixel value or index value. As notedabove, the string of like-valued pixel or index values may be referredto herein as a run. In an example for purposes of illustration, if twoconsecutive pixels or indices in a given scan order have differentvalues, the run is equal to zero. If two consecutive pixels or indicesin a given scan order have the same value but the third pixel or indexin the scan order has a different value, the run is equal to one. Forthree consecutive indices or pixels with the same value, the run is two,and so forth. Video decoder 300 may obtain the syntax elementsindicating a run from an encoded bitstream and use the data to determinethe number of consecutive locations that have the same pixel or indexvalue.

The number of indices that may be included in a run may be impacted bythe scan order. For example, consider a raster scan of lines 2660, 2680,and 2700 of map 2400. Assuming a horizontal, left to right scandirection (such as a raster scanning order), row 2660 includes threeindex values of “1,” two index values of “2,” and three index values of“3.” Row 2680 includes five index values of “1” and three index valuesof “3.” In this example, for row 2660, video encoder 200 may encodesyntax elements indicating that the first value of row 2660 (theleftmost value of the row) is 1 with a run of 2, followed by an indexvalue of 2 with a run of 1, followed by an index value of 3 with a runof 2. Following the raster scan, video encoder 200 may then begin codingrow 2680 with the leftmost value. For example, video encoder 200 mayencode syntax elements indicating that the first value of row 2680 is 1with a run of 4, followed by an index value of 3 with a run of 2. Videoencoder 200 may proceed in the same manner with line 2700.

Hence, in the raster scan order, the first index of a current line maybe scanned directly after the last index of a previous line. However, insome examples, it may not be desirable to scan the indices in a rasterscan order. For instance, it may not be desirable to scan the indices ina raster scan order where a first line of a block of video data (e.g.,row 2660) includes a first pixel adjacent to a first edge of the blockof video data (e.g., the left most pixel of row 2660, which has an indexvalue of 1) and a last pixel adjacent to a second edge of the block ofvideo data (e.g., the right most pixel of row 2660, which has an indexvalue of 3), a second line of the block of video data (e.g., row 2680)includes a first pixel adjacent to the first edge of the block of videodata (e.g., the left most pixel of row 2680, which has an index valueof 1) and a last pixel adjacent to the second edge of the block of videodata (e.g., the right most pixel of row 2680, which has an index valueof 3), the last pixel of the first line is adjacent to the last pixel ofthe second line, and the first edge and the second edge are parallel,and the last pixel in the first line has the same index value as thelast pixel in the second line, but has a different index value from thefirst pixel in the second line. This situation (i.e., where the indexvalue of last pixel in the first line is the same as the last pixel inthe second line, but different from the first pixel in the second line)may occur more frequently in computer generated screen content thanother types of video content.

In some examples, video encoder 200 may utilize a snake scan order(e.g., a traverse scan order) when encoding the indices of the map. Forinstance, video encoder 200 may scan the last pixel of the second linedirectly after the last pixel of the first line. In this way, videoencoder 200 may improve the efficiency of run-length coding.

For example, as opposed to using a raster scan order, video encoder 200may use a snake scan order to code the values of map 2400. In an examplefor purposes of illustration, consider rows 2660, 2680, and 2700 of map2400. Using a snake scan order (such as a snake scanning order), videoencoder 200 may code the values of map 2400 beginning with the leftposition of row 2660, proceeding through to the right most position ofrow 2660, moving down to the left most position of row 2680, proceedingthrough to the left most position of row 2680, and moving down to theleft most position of row 2700. For instance, video encoder 200 mayencode one or more syntax elements indicating that the first position ofrow 2660 is one and that the next run of two consecutive entries in thescan direction are the same as the first position of row 2660.

Video encoder 200 may encode one or more syntax elements indicating thatthe next position of row 2660 (i.e., the fourth position, from left toright) is two and that the next consecutive entry in the scan directionare the same as the fourth position of row 2660. Video encoder 200 mayencode one or more syntax elements indicating that the next position ofrow 2660 (i.e., the sixth position) is three and that the next run offive consecutive entries in the scan direction are the same as the sixthposition of row 2660. Video encoder 200 may encode one or more syntaxelements indicating that the next position in the scan direction (i.e.,the fourth position of row 268, from right to left) of row 2680 is oneand that the next run of nine consecutive entries in the scan directionare the same as the fourth position of row 2680.

In this way, by using a snake scan order, video encoder 200 may encodelonger length runs, which may improve coding efficiency. For example,using the raster scan, the final run of row 2660 (for the index value 3)is equal to 2. Using the snake scan, however, the final run of row 2660extends into row 2680 and is equal to 5.

Video decoder 300 may receive the syntax elements described above andreconstruct rows 2660, 2680, and 2700. For example, video decoder 300may obtain, from an encoded bitstream, data indicating an index valuefor a position of map 2400 currently being coded. Video decoder 300 mayalso obtain data indicating the number of consecutive positions in thescan order having the same index value.

As discussed above, the video coder may code the indices of the paletteindex map using horizontal and vertical traverse scans (e.g., as shownin FIG. 7). In some examples, the video coder may signal a syntaxelement that explicitly indicates the scan order (e.g.,palette_transpose_flag).

The video coder may code the palette indices using two main palettesample modes: ‘INDEX’ and ‘COPY_ABOVE’. The video coder may signal whichmode is used. For instance, the mode is signaled using a flag except forthe top row when horizontal scan is used, the first column when thevertical scan is used, or when the previous mode was ‘COPY_ABOVE’. Inthe ‘COPY_ABOVE’ mode, the palette index of the sample in the row abovemay be copied. In the ‘INDEX’ mode, the palette index is explicitlysignaled. For both ‘INDEX’ and ‘COPY_ABOVE’ modes, a run value may besignaled which specifies the number pixels that are coded using the samemode.

The video coder may utilize a specific coding order for the index map.One example coding order for index map is as follows: First, the numberof index values for the CU may be signaled. This may be followed bysignaling of the actual index values for the entire CU using truncatedbinary coding. Both the number of indices as well as the index valuesmay be coded in bypass mode. This groups the index-related bypass binstogether. Then the palette mode (INDEX or COPY_ABOVE) and run aresignaled in an interleaved manner. Finally, the component escape valuescorresponding to the escape samples for the entire CU may be groupedtogether and coded in bypass mode. An additional syntax element,last_run_type_flag, may be signaled after signaling the index values.This syntax element, in conjunction with the number of indices, mayeliminate the need to signal the run value corresponding to the last runin the block.

As discussed above, the video coder may maintain a palette predictor,with maximum size equivalent to 63 in Virtual Test Model 6.0 of VVC(VTM6.0), for coding the palette table. After processing a palettecoding unit (CU), the video coder may update the palette predictor withthe palette table of the CU, which may include the entries predictedfrom the previous palette predictor and the new signaled colours, andthe predictor entries (from the previous palette predictor) which arenot used to predict the palette table will be inserted at the end of theupdated predictor until the maximum predictor size is reached. Asdiscussed above, the latter process may be referred to as palettestuffing. FIGS. 8 and 9 are conceptual diagrams illustrating palettetable derivation and updating of a palette predictor.

As shown in FIG. 8, palette table 804 may be derived from input palettepredictor 802. For instance, a video decoder may determine a binary flagfor each respective entry of input palette predictor 802 indicatingwhether the respective entry is to be included in palette table 802. Inthe example of FIG. 8, the video decoder may determine, based on theflags, that the entries with diagonal fill, i.e., hatching, are to beincluded in palette table 802. The video decoder may also receive valuesfor one or more new entries to be included in palette table 804 that arenot included in input palette predictor 802. In the example of FIG. 8,the video decoder may receive values for each of new colors 806.

As shown in FIG. 9, input palette predictor 802 may be updated based onpalette table 804 to generate updated palette predictor 902. Forinstance, the video decoder may generate updated palette predictor 902by placing the entries of palette table 804 at the beginning and then“stuffing” the palette predictor with entries from input palettepredictor 802 (other than those already included in palette table 804)until a size of updated palette predictor 902 reaches a maximumpredictor size. As shown in FIG. 9, not all entries of input palettepredictor 802 may be included in updated palette predictor 902.

The palette stuffing, however, may take multiple cycles to completebecause the video coder (e.g., encoder/decoder) may need to checksequentially whether each entry in the predictor is used to predict thepalette table. After checking predictor entries, filling in the updatedcolor entries will also be sequential process. Since the maximum palettepredictor size currently set in Virtual Test Model 6.0 of VVC (VTM6.0)is 63 and the predictors have to be updated before encoding/decoding thenext palette coded coding units, for small blocks such as 4×4, 4×8, or8×4, the palette predictor update process can be a complex, resourceintensive procedure, which may result in a bottleneck and introducelatency in the coding pipeline.

This disclosure describes several techniques for reducing the complexityof palette-mode coding, such as palette predictor updating. Thesetechniques may be of particular benefit for small coding units.

In accordance with a first technique of this disclosure, a video codermay predict a palette predictor from a restricted (e.g., limited) set ofentries. For instance, the entries in palette predictor that can be usedin predicting palette table may be restricted.

In a first example of the first technique, for coding unit (CU) of sizeW×H, the video coder may only use the first W×H entries in the palettepredictor for predicting the palette table in the CU. FIG. 10 is aconceptual diagram illustrating an example of using only the first W×Hentries in a palette predictor for predicting the palette predictor. Asa result, the number of predictor entries that the encoder or decoderneeds to check (whether the element is used for prediction or not) forpalette stuffing is reduced to W×H. For predictor entries after theW×H'th position (entries with vertical striped fill, i.e., hatching, inFIG. 10), since the entries are not used for prediction, the elementscan be directly copied until the maximum palette size is reached.

In a second example of the first technique, besides restricting theelements that can be used for prediction, the number of elements updatedin the predictor is also restricted. For example, as shown in FIG. 11,for coding unit (CU) of size W×H, besides restricting the palettepredictor used for predicting to only the first W×H entries in thepalette predictor, same as the first example of the first technique, thenumber of predictor elements allowed to be updated with palette stuffingis also restricted to the first W×H elements. After the W×H's element,the predictor stays the same as the previous one (e.g., the same as theprevious predictor).

In accordance with a second technique of this disclosure, the videocoder may restrict the maximum size of a palette, such as the palettepredictor. In a first example of the second technique, the video codermay dynamically restrict the maximum size of palette predictor to asmaller value, e.g., 32 or 16 (smaller being relative to a defaultvalue). In this way, the video coder may reduce the overall cyclesneeded to complete palette predictor update. Also, in this way, thevideo coder may reduce the buffer size needed to maintain the palettepredictor. In a second example of the second technique, the video codermay restrict the maximum size of palette predictor to a smaller valuefor small coding units. For example, for block of size W×H smaller thana block size threshold (e.g., 63), the video coder may restrict themaximum size of the updated palette predictor to W×H. For instance, thevideo coder may selectively restrict the maximum size of a palettepredictor based on a block size.

In accordance with a third technique of this disclosure, a video codermay omit or bypass palette predictor stuffing for small coding units. Asa first example of the third technique, the video coder may not updatethe palette predictor for coding units (CUs) of size smaller than themaximum palette predictor size (currently 63 in VTM6.0) or for CUs ofsize smaller than the current palette predictor size. When the palettepredictor is not updated before coding a current CU, the video coder mayutilize the same palette predictor as for a previous CU. As a secondexample of the third technique, the video coder may update the palettepredictor to be the palette table of the CU for coding units of sizesmaller than the maximum palette predictor size (currently 63 in VTM6.0)the palette predictor.

FIG. 12 is a flowchart illustrating an example method for encoding acurrent block. The current block may comprise a current CU. Althoughdescribed with respect to video encoder 200 (FIGS. 1 and 3), it shouldbe understood that other devices may be configured to perform a methodsimilar to that of FIG. 12.

In this example, video encoder 200 initially predicts the current block(350). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate a residual blockfor the current block (352). To calculate the residual block, videoencoder 200 may calculate a difference between the original, uncodedblock and the prediction block for the current block. Video encoder 200may then transform and quantize coefficients of the residual block(354). Next, video encoder 200 may scan the quantized transformcoefficients of the residual block (356). During the scan, or followingthe scan, video encoder 200 may entropy encode the coefficients (358).For example, video encoder 200 may encode the coefficients using CAVLCor CABAC. Video encoder 200 may then output the entropy coded data ofthe block (360).

FIG. 13 is a flowchart illustrating an example method for decoding acurrent block of video data. The current block may comprise a currentCU. Although described with respect to video decoder 300 (FIGS. 1 and4), it should be understood that other devices may be configured toperform a method similar to that of FIG. 13.

Video decoder 300 may receive entropy coded data for the current block,such as entropy coded prediction information and entropy coded data forcoefficients of a residual block corresponding to the current block(370). Video decoder 300 may entropy decode the entropy coded data todetermine prediction information for the current block and to reproducecoefficients of the residual block (372). Video decoder 300 may predictthe current block (374), e.g., using an intra- or inter-prediction modeas indicated by the prediction information for the current block, tocalculate a prediction block for the current block. Video decoder 300may then inverse scan the reproduced coefficients (376), to create ablock of quantized transform coefficients. Video decoder 300 may theninverse quantize and inverse transform the coefficients to produce aresidual block (378). Video decoder 300 may ultimately decode thecurrent block by combining the prediction block and the residual block(380).

FIG. 14 is a flowchart illustrating an example method for coding a blockusing palette-mode compression, in accordance with one or moretechniques of this disclosure. Although described with respect to videodecoder 300 (FIGS. 1 and 4), it should be understood that other devicesmay be configured to perform a method similar to that of FIG. 14. Forinstance, video encoder 200 may perform the method of FIG. 14.

Video decoder 300 may determine, based on a parameter of a first blockof video data, a maximum number of entries to be used for palette-modecoding of the current block (1402). For instance, palette predictionunit 319 of video decoder 300 may determine, based on a value of theparameter, whether to restrict the maximum number of entries to be 16 or32. In some examples, the parameter may be representative of a size ofthe first block, e.g., as signaled by an encoder in the encodedbitstream. For instance, palette prediction unit 319 may restrict themaximum number of entries to be W×H where W is a width of the firstblock and H is a height of the first block. As such, where the firstblock is 4×4, palette prediction unit 319 may restrict the maximumnumber of entries to be 16. Similarly, where the first block is 4×8,palette prediction unit 319 may restrict the maximum number of entriesto be 32. The determined maximum number of entries may be a maximumnumber of entries in a palette constructed for the first block, apalette predictor used to construct the palette, or any other paletteused for coding of the first block.

Video decoder 300 may generate, based on the determined maximum numberof entries and based on a palette predictor, a palette for the firstblock of video data (1404). The generated palette may include one ormore entries each including a palette index that is associated with acolor value. To generate the palette based on the determined maximumnumber of entries, palette prediction unit 319 may limit a number ofentries in the palette to the determined maximum number or generate thepalette from a palette predictor that includes a number of entries thatis limited to the determined maximum number. By limiting the number ofentries in the palette to the determined maximum number, the palette mayhave a number of entries that is less than or equal to the determinedmaximum number. Similarly, where the palette is generated from a palettepredictor that includes a number of entries that is limited to thedetermined maximum number, the palette predictor may have a number ofentries that is less than or equal to the determined maximum number.

Video decoder 300 may decode, from a coded video bitstream and for thefirst block of video data, index values for samples of the first blockthat identify entries in the palette (1406). For instance, entropydecoding unit 302 may decode from the encoded video bitstream, andprovide to palette prediction unit 319, an array of index values.

Video decoder 300 may reconstruct, based on the index values, thesamples of the first block (1408). For instance, palette prediction unit319 may use the generated palette as a look-up table to translate theindex values into color values (e.g., as discussed above with referenceto FIGS. 5-7) for respective samples.

Video decoder 300 may update, based on the generated palette, the inputpalette predictor to generate an updated palette predictor. Forinstance, palette prediction unit 319 may generate the updated palettepredictor as discussed above with reference to FIGS. 8-11. As oneexample, palette prediction unit 319 may copy, to the updated palettepredictor, entries from the palette for the first block; and copy, tothe updated palette predictor and at a position that is after theentries from the palette for the first block, entries from a restrictedportion of the input palette predictor that are not included in thepalette for the first block. In this example, palette prediction unit319 may copy, to the updated palette predictor and at a position that isafter the entries from the restricted portion of the input palettepredictor, entries from an unrestricted portion of the input palettepredictor until a maximum size of the updated palette predictor isreached. Copying entries from the restriction portion may includecopying a limited number of entries from the restricted portion.

As another example, palette prediction unit 319 may copy, to the updatedpalette predictor, entries from the palette for the first block; andstuff the updated palette predictor with entries of the input palettepredictor that are not included in the palette for the first block. Insome examples, to stuff the palette, palette prediction unit 319 maystuff the updated palette predictor in response to determining that asize of the first block is greater than a threshold size. Similarly,palette prediction unit 319 may refrain from stuffing the updatedpalette predictor in response to determining that the size of the firstblock is less than the threshold size.

Video decoder 300 may generate, based on the updated palette predictor,a palette for a second block of video data. For instance, video decoder300 may copy one or more entries from the updated palette predictor intothe palette for the second block of video data.

The following numbered examples may illustrate one or more aspects ofthe disclosure:

Example 1. A method of coding video data, the method comprising:obtaining an input palette predictor; generating, based on the inputpalette predictor, a palette for a first block of video data; updating,based on the generated palette, the input palette predictor to generatean updated palette predictor, wherein updating the input palettepredictor comprises: copying, to the updated palette predictor, entriesfrom the palette for the first block; and copying, to the updatedpalette predictor and at a position that is after the entries from thepalette for the first block, entries from a restricted portion of theinput palette predictor that are not included in the palette for thefirst block; and generating, based on the updated palette predictor, apalette for a second block of video data.

Example 2. The method of example 1, wherein updating the input palettepredictor further comprises: copying, to the updated palette predictorand at a position that is after the entries from the restricted portionof the input palette predictor, entries from an unrestricted portion ofthe input palette predictor until a maximum size of the updated palettepredictor is reached.

Example 3. The method of any of examples 1 or 2, wherein the entriesfrom the restricted portion comprises: copying a limited number ofentries from the restricted portion.

Example 4. A method of coding video data, the method comprising:obtaining an input palette predictor, wherein a size of the inputpalette predictor is restricted to a value less than 63; generating,based on the input palette predictor, a palette for a first block ofvideo data; updating, based on the generated palette, the input palettepredictor to generate an updated palette predictor; and generating,based on the updated palette predictor, a palette for a second block ofvideo data.

Example 5. The method of example 4, wherein the value is 32.

Example 6. The method of example 4, wherein the value is 16.

Example 7. The method of any of examples 4-6, further comprising:restricting the size of the input palette predictor responsive todetermining that a size of the first block is less than a thresholdblock size.

Example 8. The method of example 7, wherein restricting the size of theinput palette predictor comprises restricting the size of the inputpalette predictor to a size equal to a length times a width of the firstblock.

Example 9. A method of coding video data, the method comprising:obtaining an input palette predictor; generating, based on the inputpalette predictor, a palette for a first block of video data; updating,based on the generated palette, the input palette predictor to generatean updated palette predictor, wherein updating the input palettepredictor comprises: copying, to the updated palette predictor, entriesfrom the palette for the first block; and stuffing the updated palettepredictor with entries of the input palette predictor that are notincluded in the palette for the first block; and generating, based onthe updated palette predictor, a palette for a second block of videodata.

Example 10. The method of example 9, wherein stuffing the palettecomprises stuffing the updated palette predictor in response todetermining that a size of the first block is greater than a thresholdsize.

Example 11. The method of example 10, further comprising not stuffingthe updated palette predictor in response to determining that the sizeof the first block is less than the threshold size.

Example 12. A method comprising the method of any of examples 1-11.

Example 13. The method of any of examples 1-12, wherein the first blockis a first coding unit (CU) of video data.

Example 14. The method of any of examples 1-13, wherein coding comprisesdecoding.

Example 15. The method of any of examples 1-14, wherein coding comprisesencoding.

Example 16. A device for coding video data, the device comprising one ormore means for performing the method of any of examples 1-15.

Example 17. The device of example 16, wherein the one or more meanscomprise one or more processors implemented in circuitry.

Example 18. The device of any of examples 16 and 17, further comprisinga memory to store the video data.

Example 19. The device of any of examples 16-18, further comprising adisplay configured to display decoded video data.

Example 20. The device of any of examples 16-19, wherein the devicecomprises one or more of a camera, a computer, a mobile device, abroadcast receiver device, or a set-top box.

Example 21. The device of any of examples 16-20, wherein the devicecomprises a video decoder.

Example 22. The device of any of examples 16-21, wherein the devicecomprises a video encoder.

Example 23. A computer-readable storage medium having stored thereoninstructions that, when executed, cause one or more processors toperform the method of any of examples 1-15.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the terms “processor” and “processingcircuitry,” as used herein may refer to any of the foregoing structuresor any other structure suitable for implementation of the techniquesdescribed herein. In addition, in some aspects, the functionalitydescribed herein may be provided within dedicated hardware and/orsoftware modules configured for encoding and decoding, or incorporatedin a combined codec. Also, the techniques could be fully implemented inone or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A device for decoding video data, the devicecomprising a memory configured to store at least a portion of an encodedvideo bitstream; and one or more processors that are implemented incircuitry and configured to: determine, based on a parameter of a firstblock of video data, a maximum number of entries to be used forpalette-mode coding of the current block; generate, based on thedetermined maximum number of entries and based on a palette predictor, apalette for the first block of video data, the palette including one ormore entries each including a palette index that is associated with acolor value; decode, from the encoded video bitstream and for the firstblock of video data, index values for samples of the first block thatidentify entries in the palette; and reconstruct, based on the indexvalues, the samples of the first block.
 2. The device of claim 1,wherein the maximum number of entries is restricted to a value less than63.
 3. The device of claim 2, wherein, to determine the maximum numberof entries, the one or more processors are configured to: restrict themaximum number of entries to be 16 responsive to the parameter having afirst value; and restrict the maximum number of entries to be 32responsive to the parameter having a second value different than thefirst value.
 4. The device of claim 1, wherein the one or moreprocessors are further configured to: obtain an input palette predictor,wherein, to generate the palette, the one or more processors areconfigured to generate the palette based on the input palette predictor;update, based on the generated palette, the input palette predictor togenerate an updated palette predictor, wherein, to update the inputpalette predictor, the one or more processors are configured to: copy,to the updated palette predictor, entries from the palette for the firstblock; and copy, to the updated palette predictor and at a position thatis after the entries from the palette for the first block, entries froma restricted portion of the input palette predictor that are notincluded in the palette for the first block; and generate, based on theupdated palette predictor, a palette for a second block of video data.5. The device of claim 4, wherein, to update the input palettepredictor, the one or more processors are configured to: copy, to theupdated palette predictor and at a position that is after the entriesfrom the restricted portion of the input palette predictor, entries froman unrestricted portion of the input palette predictor until a maximumsize of the updated palette predictor is reached.
 6. The device of claim5, wherein, to copy the entries from the restricted portion, the one ormore processors are configured to: copy a limited number of entries fromthe restricted portion.
 7. The device of claim 1, wherein the one ormore processors are further configured to: obtain an input palettepredictor, wherein, to generate the palette, the one or more processorsare configured to generate the palette based on the input palettepredictor; update, based on the generated palette, the input palettepredictor to generate an updated palette predictor, wherein, to updatethe input palette predictor, the one or more processors are configuredto: copy, to the updated palette predictor, entries from the palette forthe first block; and stuff the updated palette predictor with entries ofthe input palette predictor that are not included in the palette for thefirst block; and generate, based on the updated palette predictor, apalette for a second block of video data.
 8. The device of claim 7,wherein, to stuff the palette, the one or more processors are configuredto stuff the updated palette predictor in response to determining that asize of the first block is greater than a threshold size.
 9. The deviceof claim 8, wherein, to stuff the palette, the one or more processorsare configured to not stuff the updated palette predictor in response todetermining that the size of the first block is less than the thresholdsize.
 10. The device of claim 1, further comprising a display configuredto output the reconstructed first block of video data.
 11. A method ofdecoding video data, the method comprising: determining, based on aparameter of a first block of video data, a maximum number of entries tobe used for palette-mode coding of the current block; generating, basedon the determined maximum number of entries and based on a palettepredictor, a palette for the first block of video data, the paletteincluding one or more entries each including a palette index that isassociated with a color value; decoding, from an encoded video bitstreamand for the first block of video data, index values for samples of thefirst block that identify entries in the palette; and reconstructing,based on the index values, the samples of the first block.
 12. Themethod of claim 11, wherein the maximum number of entries is restrictedto a value less than
 63. 13. The method of claim 12, wherein determiningthe maximum number of entries comprises selectively restricting themaximum number of entries to be either 16 or
 32. 14. The method of claim11, further comprising: obtaining an input palette predictor, whereingenerating the palette comprises generating the palette based on theinput palette predictor; updating, based on the generated palette, theinput palette predictor to generate an updated palette predictor,wherein updating the input palette predictor comprises: copying, to theupdated palette predictor, entries from the palette for the first block;and copying, to the updated palette predictor and at a position that isafter the entries from the palette for the first block, entries from arestricted portion of the input palette predictor that are not includedin the palette for the first block; and generating, based on the updatedpalette predictor, a palette for a second block of video data.
 15. Themethod of claim 14, wherein updating the input palette predictor furthercomprises: copying, to the updated palette predictor and at a positionthat is after the entries from the restricted portion of the inputpalette predictor, entries from an unrestricted portion of the inputpalette predictor until a maximum size of the updated palette predictoris reached.
 16. The method of claim 15, wherein copying the entries fromthe restricted portion comprises: copying a limited number of entriesfrom the restricted portion.
 17. The method of claim 11, furthercomprising: obtaining an input palette predictor, wherein generating thepalette comprises generating the palette based on the input palettepredictor; updating, based on the generated palette, the input palettepredictor to generate an updated palette predictor, wherein updating theinput palette predictor comprises: copying, to the updated palettepredictor, entries from the palette for the first block; and stuffingthe updated palette predictor with entries of the input palettepredictor that are not included in the palette for the first block; andgenerating, based on the updated palette predictor, a palette for asecond block of video data.
 18. The method of claim 17, wherein stuffingthe palette comprises stuffing the updated palette predictor in responseto determining that a size of the first block is greater than athreshold size.
 19. The method of claim 18, further comprising notstuffing the updated palette predictor in response to determining thatthe size of the first block is less than the threshold size.
 20. Adevice for encoding video data, the device comprising a memoryconfigured to store at least a portion of an encoded video bitstream;and one or more processors that are implemented in circuitry andconfigured to: determine, based on a parameter of a first block of videodata, a maximum number of entries to be used for palette-mode coding ofthe current block; generate, based on the determined maximum number ofentries and based on a palette predictor, a palette for the first blockof video data, the palette including one or more entries each includinga palette index that is associated with a color value; and encode, inthe encoded video bitstream and for the first block of video data, indexvalues for samples of the first block that identify entries in thepalette.
 21. The device of claim 20, wherein the maximum number ofentries is restricted to a value less than
 63. 22. The device of claim21, wherein, to determine the maximum number of entries, the one or moreprocessors are configured to: restrict the maximum number of entries tobe 16 responsive to the parameter having a first value; and restrict themaximum number of entries to be 32 responsive to the parameter having asecond value.
 23. A method of encoding video data, the methodcomprising: determining, based on a parameter of a first block of videodata, a maximum number of entries to be used for palette-mode coding ofthe current block; generating, based on the determined maximum number ofentries and based on a palette predictor, a palette for the first blockof video data, the palette including one or more entries each includinga palette index that is associated with a color value; and encoding, inan encoded video bitstream and for the first block of video data, indexvalues for samples of the first block that identify entries in thepalette.
 24. The method of claim 23, wherein the maximum number ofentries is restricted to a value less than
 63. 25. The method of claim24, wherein determining the maximum number of entries comprisesselectively restricting the maximum number of entries to be either 16 or32.
 26. A computer-readable storage medium having stored thereoninstructions that, when executed, cause one or more processors of avideo decoder to: determine, based on a parameter of a first block ofvideo data, a maximum number of entries to be used for palette-modecoding of the current block; generate, based on the determined maximumnumber of entries and based on a palette predictor, a palette for thefirst block of video data, the palette including one or more entrieseach including a palette index that is associated with a color value;decode, from an encoded video bitstream and for the first block of videodata, index values for samples of the first block that identify entriesin the palette; and reconstruct, based on the index values, the samplesof the first block.
 27. A computer-readable storage medium having storedthereon instructions that, when executed, cause one or more processorsof a video encoder to: determine, based on a parameter of a first blockof video data, a maximum number of entries to be used for palette-modecoding of the current block; generate, based on the determined maximumnumber of entries and based on a palette predictor, a palette for thefirst block of video data, the palette including one or more entrieseach including a palette index that is associated with a color value;and encode, in an encoded video bitstream and for the first block ofvideo data, index values for samples of the first block that identifyentries in the palette.